The National Weather Service late today upped the forecasted snowfall totals for major northeastern cities over the weekend: it said New York City can expect 12 inches of snow before the storm ends, and Boston may get four to eight inches.

The worst of the storm was still expected in a swath from central Virginia to New Jersey. Washington, Baltimore and Philadelphia were all forecast to get 10-20 inches of snow. The governor of Virginia has already declared a state of emergency. Washington, D.C., declared a snow emergency so that it can clear major streets and reduce mass transit service if necessary.

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Winter Storm Warnings were extended by the National Weather Service from eastern Tennessee and Kentucky to the southern coast of New England -- and blizzard warnings were issued for New York's Long Island.

"It's going to be a bad storm," said one government forecaster. "The I-95 corridor -- it's going to be a mess."

The storm's first effect has been rain in Alabama, Georgia and Florida. But the greater effect was likely to come Saturday afternoon into Sunday morning, as the storm moves toward the Atlantic coast -- colliding with the coldest air in the region since late last winter. The high temperature in New York City, for instance, was 29 degrees today, with wind chills in the teens.

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Roanoke, Va., was forecast to get 10-15 inches of snow Friday night into Saturday. Washington, Baltimore and Philadelphia were told to hunker down Saturday afternoon and into the night.

"Philadelphia, D.C., Baltimore -- that's where the thrust of the storm will be, the largest snow amounts," said Bob Oravec, lead forecaster at NOAA's National Weather Service in Camp Springs, Md.

The forecast office for the Washington-Baltimore area issued an updated Winter Storm Warning effective from midnight tonight to 6:00 a.m. Sunday:

"Snowfall is expected to be heaviest between 8 a.m. and 6 p.m. on Saturday. Conditions will deteriorate rapidly Saturday morning and will make travel extremely treacherous during the day into the evening hours on Saturday."

The economic effects might be as big as the inconvenience.

"A snowstorm like this is not good news," said Scott Burnhardt of weather research firm Planalytics. "This is 'Super Saturday,' we're talking about well over $1 billion traded on this day."

Delta Airlines canceled all its Saturday flights from Washington, Philadelphia, and Richmond, Va., to prevent planes from getting stuck at snowed-in airports. "Customers may wish to consider postponing or rerouting their trip without penalty to avoid possible inconvenience," the company said in a statement

JetBlue suspended some of its fees for last-minute reservation changes, though they were limited to Baltimore, Charlotte, N.C., Raleigh/Durham, N.C.; Richmond, Va., and Washington's Dulles Airport.

Amtrak said it did not expect delays. But it put extra crews on alert, partly, it said, to keep ice from building up on overhead power lines.

White Christmas Coming?

"Now the fun begins!" said a National Weather Service advisory from the forecast office in Mt. Holly, N.J. The center of the storm, said forecasters, was likely to reach the coast of North Carolina on Saturday morning, but after that, computer models disagreed on how close it would stay to land.

"Low pressure tracks to near Cape Hatteras Saturday morning, then deepens as it tracks to the northeast over the Atlantic Saturday night," said the Weather Service office for the New York City area. "The intense low will produce a swath of heavy snowfall...as well as a corridor of high winds. Once again...a track difference of only 100 miles will have a significant impact on the weather across the region."

Generally, snowfall in the northeastern U.S. can be expected to be heaviest along the coastline, said forecasters.

Next week, a second storm could bring a white Christmas to much of the country -- in a wide arc that could stretch from the Rockies to the mid-South to New England.

"A new surge of cold air will press southward later this weekend and lay the groundwork for snow, ice and rain over a large swath of the nation during Christmas week," said AccuWeather's Alex Sosnowski.

But the short-term forecast was of more concern. "I guess it's a good thing it's happening on a weekend," said a forecaster.

Additional information from ABC News' Lisa Stark. The Associated Press also contributed to this report.